The preferred embodiment relates to a method and a system for controlling a process fluid stream in an industrial process plant operating in the technical area of chemistry, petrol chemistry, gas processing, pharmaceutics or the production of food, cellulose, paper, glass, steel or cement or similar areas. In an industrial process plant process-engineering processes are running in which fluid streams relating to a main process are influenced continuously or discontinuously. For example, in large scale chemical plants, pharmaceutical plants, steel and cement producing plants, waste incineration plants, foundries, etc. process fluid streams are formed that are to be adjusted as required by the process and the operation in accordance with certain control parameters such as temperature or flow rate. In industrial process plants also raw materials can be extracted such as metal from ore. Therein the raw material for a process can itself be the result of a preceding process. The production of finished products or the recycling of raw materials from waste and garbage also belong to the technical area of process engineering.
For controlling a fluid stream in a line network of an industrial process plant it is generally known to employ field devices or positioners often designed as adjustable control valves. By means of a control valve the flow cross-section of a line can be adjusted in order to change the fluid stream according to the control parameters. As the positioning device of a process engineering industrial process plant is often located in environments at risk of explosion, the positioning device is actuated by means of a pneumatic drive controlled by a positioner with an current/pressure converter (I/P converter). The positioner receives external required positioning signals in order to position the positioning device according to the control parameters and thereby control the process fluid stream. Furthermore, the positioner receives actual position signals from a position sensor and contains its own control routines in order to produce a positioning signal for the pneumatic drive based on the desired required value and the measured position. A disadvantage of controlling a process fluid stream by means of a control valve is the control valve acts as a variable flow resistance and the narrower the flow cross-section limited by the control valve becomes, the larger becomes the pressure difference present at the positioning member and the larger the loss in flow rate to be tolerated. It results that each positioning device has a specific functional characteristic and thereby exhibits a specific control behavior. A particularly important control parameter is the so-called valve authority, which relates to the ability of the control valve to precisely control and adjust the desired process fluid stream and which for example can be defined by the ratio of the process fluid pressure difference at the positioning valve when flow is present and the pressure difference for minimum flow according to experience. When the pressure difference at the control valve becomes too small the ability to precisely control and adjust the flow disappears. Therefore, in order to provide a functional control it is necessary to assure a sufficient pressure difference at the control valve i.e. to assure that valve authority is present.
A pump is provided for producing the process fluid stream within the line network of the industrial process plant, wherein for reasons of costs and control efficiency the pump commonly provides a constant fluid pressure for flow generation. The drive value such as the rotation speed of the pump of an industrial process plant is generally not variable because influencing the process fluid stream by means of the pump in a fluid-stream-user-specific manner is difficult, in particular if several users of streams are arranged downstream of the pump at different line sections. In any aspect the control effort for a rotation-controlled pump would be very elaborate.
Recently there are development tendencies in the area of industrial process plants to completely replace control valves by a pump with rotation speed control individually adapted to the process consumer. For example, DE 10 2007 053 948 A1 discloses an installation for controlling a fluid stream in a heater, wherein instead of a control valve the pump of the industrial process installation is a motor-driven centrifugal pump adjustable according to demand.
However, as indicated above, it results that a control system adapted for individual process users where control valves are replaced by rotation speed control is—if possible at all—only feasible with an extremely high design and control effort and energy expenditure.